In fume hood control systems, and in other applications where it is desired to accurately control air flow through a duct or other channel (hereinafter "duct"), Venturi or other air flow valves are frequently utilized. With such valves, there is a known and predictable relationship between the position of a valve control shaft and air flow through the valve.
While such valves provide effective air flow control, one problem has been that the ducts in which the valves are utilized are not of uniform size and shape. Since a single small valve mounted in a large duct cannot handle desired air flow through the duct, two techniques have heretofore been utilized for larger ducts, neither of which is completely satisfactory.
The first technique is to provide larger valves for the larger ducts. While this has the advantage of permitting a single actuator to control air flow in the same manner as for smaller ducts, large valves become long and unwieldly to work with. They can also be relatively heavy. In particular, because of the added length required for the larger valve, one 16 inch valve can weigh as much and take up as much area as two 12 inch valves. Since large valves also tend to magnify small errors, more precise manufacturing tolerances are required for such valves, making them more difficult and expensive to manufacture. In addition, Venturi valves normally have a generally circular cross section and therefore generally fit well in ducts which are substantially square or circular. However, because of space limitations in buildings, large ducts are typically rectangular, making it difficult to fit a large valve in such ducts. Finally, it is substantially less expensive to build one or two valve modules in selected sizes and to gang such modules to fill larger ducts than it is to build valves in all of the various sizes which might be required for different duct sizes.
However, ganging two or more smaller valves to fill a duct opening also presents some problems. One problem is that, with current designs, the position of each valve needs to be independently measured and a separate actuator provided to drive the valve. Separate electronic feedback controls are also required for each valve/actuator pair. This arrangement, with multiple actuators, is expensive, and is difficult to install and maintain. It also requires additional testing and calibration to assure that the various valve/actuator combinations are operating in synchronism.
This suggests that the advantages of a multi valve system for large ducts can be achieved without the disadvantages by providing a single actuator and a single control for all of the valves (or at least for selected numbers of the valves). In doing this, advantage is taken of the fact that the air flow through a given valve for a given valve shaft position is a constant which can be determined, so that if the shaft position for one valve is measured and the valves can be tied together so as to operate in tandem, it should be possible to use the single valve measurement to control the actuator driving all of the valves. However, in practice, it has been found difficult to design a mechanism for linking the valves to the actuator in a manner such that the valves move in tandem. In particular, the moment arms of such linkages tend to permit linkage skewing, resulting in uneven movement of the valves and thus in poor control of air flow. While efforts have been made to stiffen the linkages between the actuator and the valves, such efforts have not heretofore been completely successful, the linkages either not being stiff enough or being too bulky and cumbersome to be used effectively. As a result, use of ganged valves from a single actuator has not heretofore been commercially practical.
A need therefore exists for an improved mechanism for linking a single actuator to operate a plurality of valves, which mechanism assures that the valves move in tandem so that a valve position measured for one of the valves can be utilized to control the actuator to provide a desired precise air flow through the ganged valve combination.